The reduction of vehicle body weights has been a critical issue, which relates to the regulation of emission gas and recent global environmental problems. In order to lighten the body of a vehicle, it is effective to reduce the thickness of steel sheets by increasing the strength of steel sheets that are used in quantity, in other words, by using high tensile strength steel sheets.
However, even vehicle parts of thin high tensile strength steel sheets have to perform sufficiently well based on their purposes. The performance includes, for instance, static strength against bending and torsional deformation, fatigue resistance, impact resistance, and the like. Therefore, high tensile strength steel sheets for use in vehicle parts also have to have such excellent characteristics after being formed.
Moreover, press forming is carried out on steel sheets to form vehicle parts. However, when the steel sheets are too strong, the following problems are found:    (1) shape freezability declines; and    (2) problems such as cracking and necking are found during forming due to a decrease in ductility. The application of high tensile strength steel sheets to vehicle bodies has been limited.
In order to overcome this problem, steel sheets that use an extra-low carbon steel as a material and in which the amount of C finally remaining in a solid solution state is controlled in an appropriate range are known as, for instance, cold rolled steel sheets for an outer sheet panel. This type of steel sheet is kept soft during press forming, and maintains shape freezability and ductility and maintains dent resistance due to an increase in yield stress which utilized strain age hardening phenomenon during the coating and baking process of 170° C.×about 20 minutes after press forming. In this type of steel sheet, C is dissolved in steel in a solid solution state during press forming, and the steel is soft. On the other hand, after press forming, solid solution C is fixed to a dislocation that is introduced during the press forming, in the coating and baking process, thus increasing yield stress.
However, an increase in yield stress due to strain age hardening is kept low in this type of steel sheet in order to prevent stretcher strains that will later become surface defects. Thus, there is little contribution to the actual weight reduction of parts.
Specifically, not only does yield stress have to be increased by strain aging but strength characteristics also have to increase so as to reduce the weight of parts. In other words, it is desirable to make parts stronger by increasing tensile strength after strain aging.
For applications in which appearance is not so much of a concern, proposed are steel sheets in which a baking hardening quantity is further increased by using solid solution N, and steel sheets which have a composite structure consisting of ferrite and martensite and thus have improved baking hardenability.
For instance, Japanese Unexamined Patent Application Publication No. 60-52528 discloses a production of high-strength thin steel having good ductility and spot weldability in which steel containing 0.02 to 0.15% of C, 0.8 to 3.5% of Mn, 0.02 to 0.15% of P, 0.10% or less of Al, and 0.005 to 0.025% of N is coiled at 550° C. or below for hot-rolling, and annealing after cool-rolling is a controlled cooling heat treatment. The steel sheet produced in the art of Japanese Unexamined Patent Application Publication No. 60-52528 has a mixed structure consisting of a low-temperature transformation product phase mainly having ferrite and martensite, and has excellent ductility. At the same time, high strength is obtained by utilizing strain aging during a coating and baking process due to N, which is actively added.
However, in the art of Japanese Unexamined Patent Application Publication No. 60-52528, an increase in yield stress YS due to strain age hardening is large, but an increase in tensile strength TS is small. Moreover, the fluctuation of mechanical properties is large, so that an increase in yield stress YS is large and uneven. Thus, it is not currently possible to expect a steel sheet that is thin enough to contribute the weight reduction of vehicle parts.
Moreover, Japanese Examined Patent Application Publication No. 5-24979 discloses a cold rolled high tensile steel sheet having baking hardenability. The steel sheet contains 0.08 to 0.20% of C and 1.5 to 3.5% of Mn, and the balance Fe and inevitable impurities as components. The steel structure is composed of uniform bainite containing 5% or less of ferrite, or bainite partly containing martensite. In the cold rolled steel sheet described in Japanese Examined Patent Application Publication No. 5-24979, a baking hardening quantity, as a structure mainly having bainite, is greater than conventionally used due to quenching in the temperature range of 400 to 200° C. and the following slow cooling in a cooling process after continuous annealing.
However, although a baking hardening quantity is greater than conventionally used due to an increase in yield strength after coating and baking in the cold rolled steel sheet described in Japanese Examined Patent Application Publication No. 5-24979, tensile strength cannot be increased. When the steel sheet is used for strong members, the improvement of fatigue resistance and impact resistance cannot be expected. Thus, there still is a problem in that the steel sheet cannot be used for applications that strongly require fatigue resistance, impact resistance, and the like.
Although it is a hot rolled steel sheet, proposed is a steel sheet having higher yield stress as well as yield strength due to a heat treatment after press forming.
For instance, Japanese Examined Patent Application Publication No. 8-23048 proposes a production of hot rolled steel plate having a composite structure mainly of ferrite and martensite in which steel containing 0.02 to 0.13% of C, 2.0% or less of Si, 0.6 to 2.5% of Mn, 0.10% or less of sol. Al, and 0.0080 to 0.0250% of N is reheated at 1,100° C. or higher and finish rolling is finished at 850 to 900° C. for hot-rolling. Then, the steel is cooled to less than 150° C. at the cooling rate of 15° C./s or higher, and is coiled. However, although yield stress as well as tensile strength increase due to strain age hardening in the steel sheet produced in the art described in Japanese Examined Patent Application Publication No. 8-23048, steel is coiled at an extremely low coiling temperature of less than 150° C. Thus, the inconsistency of mechanical characteristics is large and troublesome. There also have been problems in that increases in yield stress after a press forming-coating and baking treatment are uneven, and furthermore, a hole expanding ratio (λ) is low, so that stretch-flanging workability declines and press forming becomes insufficient.
High tensile strength steel sheets having relatively high yield stress include so-called precipitation strengthened steel to which carbonitride-forming elements, such as Ti, Nb and V, are added and which is strengthened by the fine deposits thereof. However, unlike hot rolled steel sheets that go through a sufficient thermal insulation process after hot rolling, it is difficult for cold rolled steel sheets to obtain enough precipitation in a short period of continuous annealing. It has been difficult to produce a steel sheet having high yield ratios (ratios of yield stress relative to tensile strength:. YS/TS). Particularly, when C is reduced for weldability, it becomes more difficult to have high yield ratios, probably because the amount of deposit itself decreases in a region where the amount of C is low, and this is troublesome.
Furthermore, although the above-mentioned steel sheets show excellent strength after a coating and baking treatment in a simple tensile test, strengths are largely uneven when plastic deformation is carried out under actual press conditions. The steel sheets are not sufficiently applicable for parts that need to be reliable.
It is an object of the present invention to break through the limitations of the conventional arts mentioned above, and to provide a high tensile strength cold rolled steel sheet having excellent strain age hardening characteristics, high formability and stable quality and thus can obtain sufficient strength after being formed into vehicle parts, fully contributing to the reduction of vehicle body weights, and the production thereof that can economically produce the steel sheets without distorting the shapes thereof. The strain age hardening characteristics in the present invention target 80 MPa or more of BH amounts and 40 MPa or more of ΔTS under the aging condition of holding the temperature at 170° C. for 20 minutes after predeformation at 5% of tensile strain.
Furthermore, the steel sheet is also advantageously applicable to, particularly, parts to which relatively small strain is added. Thus, it is also an object of the present invention to provide a high tensile strength cold rolled steel sheet having high yield ratios of 0.7 or higher so as to raise sheet yield stress and stabilize the strength of parts.